Over the past decade, there has been a move to minimize the risk for such complications by reducing the intensity of conditioning regimens. Added impetus for reducing conditioning intensity arose from the observation in transplantation for thalassemia and sickle cell disease that sustained mixed chimerism, that is partial donor engraftment, is usually sufficient to cure non-malignant diseases. This observation suggested that sustained engraftment could be achieved without "ablation" or elimination of the host marrow. Pre-clinical studies demonstrated in small and large animals that sustained mixed chimerism can be achieved with preparative regimens consisting of TBI doses as low as 100-300 cGy (by comparison, standard intensity regimens typically employ 1000 cGy or more in combination with chemotherapy).